Field of the Invention and Related Art Statement
The present invention relates to an apparatus for detecting or sensing an operating condition of an internal-combustion engine, and more particularly to an apparatus for detecting an abnormal combustion such as so-called knocking of an internal-combustion engine having ignition plugs.
Nowadays various types of internal-combustion engines are installed in automobiles. One, the most widely used engine, is a four stroke cycle gasoline engine having ignition plugs. In this gasoline, engine, in order to improve the output power characteristic and fuel economy of the engine, the ignition timing is advanced with respect to the top dead center by such an angle that the maximum torque can be obtained. In general, such an advanced ignition timing is called minimum advance for best torque (M.B.T.) However, M.B.T., i.e., the most suitable ignition timing or the latest ignition timing for obtaining maximum torque under the same running conditions, varies in accordance with running conditions of the automobile and operating conditions of the engine. For instance, in a relatively slow engine revolution range, a threshold ignition timing at which the knocking or detonation might occur lies at a position which is delayed with respect to the M.B.T., so that if the ignition timing is set at M.B.T., knocking occurs and the output power and fuel consumption are deteriorated. In view of the above it has been earnestly desired to develop an apparatus which can sense the occurrence of abnormal conditions of the engine, particularly knocking, accurately.
In Japanese Patent Application Publication No. 5154/41, there is disclosed a known knocking sensor comprising a pressure sensor such as a piezoelectric element. The piezoelectric element is shaped as a washer and is installed in an ignition plug to sense pressure variation within a combustion chamber of the engine. An electric signal generated by the piezoelectric element is supplied by means of an electric conductor to a signal processing circuit in which the pressure variation is detected.
There has been proposed another knocking sensor which comprises a vibration sensor formed by a piezoelectric element. An electric signal generated by the vibration sensor is supplied to a plurality of filter circuits to detect signal components corresponding to abnormal vibration due to knocking, and the thus extracted signal components are further processed by a circuit for judging knocking.
In the above mentioned known knocking sensor utilizing the pressure sensor, the pressure sensor is arranged in the combustion chamber to detect the pressure variation within the combustion chamber. In the known knocking sensor using the vibration sensor, the vibration sensor is provided on an engine block. Therefore, the sensors detect pressure variations or vibrations caused by various factors other than the combustion of the fuel within the combustion chamber. This results in that the electric signal generated by the sensor has very low S/N. Further the electric signal generated by the sensor is derived by means of an electric conductor to the signal processing circuit which is usually provided remote from the engine. It is apparent that noise might be introduced into the electric signal, while it is transmitted through the electric conductor. This results in further decrease in S/N. In this manner, in the known knocking sensors, S/N of the detected signal is very low, and thus the detected signal has to be processed by means of a very complicated signal processing circuit. Therefore, the signal processing circuit is liable to be complicated in construction and expensive in cost. Moreover, the complicated signal processing requires a longer time, which limits the response of the knocking detection. In order to control ignition timing in a real time mode in accordance with the detected engine condition, it is necessary to make the response as fast as possible.